Is there a limit to the number of markers I can add to a map

No, you can add unlimited markers to each map you create with Easy2Map.

How do I upload a custom icon for my map's markers

When adding a new marker (or editing an existing marker), click on the 'Change Icon' button, thereafter click on the 'Browse' button to browse to the image on your computer that you wish to use. Click the blue 'Upload Icon' button to upload the icon and associate it with your map marker.